Cellular mobile telephones and other such radio communications devices are usually designed to a meet the requirements of a limited number of standards in common use in the world. Many countries choose the same standard; for example the pan-European GSM system is used by 14 European countries, Australia and some Middle Eastern countries. The U.S. digital cellular standard, IS-54B Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard (available from the Telecommunications Industry Association, 2001 Pennsylvania Avenue, N.W., Washington D.C., 20006), (IS-54B) is used, for example, in the USA, Mexico, Canada and South America. Throughout the following, the terms cellular mobile telephone, cellular phone, cellular telephone, mobile telephone, `phone`, radiotelephone terminal, cellular terminal, and "terminal" may be used equivalently to refer to a wireless communications device capable of wirelessly transmitting and receiving data. Also, the terms radiotelephone network, cellular system, and cellular network are used equivalently to refer to a wireless communications system which provides wireless data connections between two or more terminals or between two or more terminals and other equipment. Cellular mobile telephones are produced in enormous volume and it is therefore desirable that the design be the same for all markets.
One problem that exists is that cellular telephone numbers, also referred to as the mobile identification number (MIN), are re-used in other countries. A particular MIN is not guaranteed to be unique in the whole world. For example, a cellular telephone brought to the US by a traveler from another country which uses the same cellular standard as is used in the U.S., may have the same telephone number as a native American phone, and can sometimes access the network in an unforeseen and unauthorized manner. Fraudulent access is also a problem. It is an object of anti-fraud systems to prevent such unauthorized access.
Anti-fraud systems make use of a secret number such as a personal identification number (PIN code) embedded into every phone. The PIN code stored in the cellular phone is also stored in the cellular telephone exchange, or network exchange, belonging to the operator with which the user has a subscription, i.e. the so called "home system." A cellular network is comprised of many such exchanges, or switches, and associated base stations. A cellular phone attempting to access the cellular network is "challenged" with a random number broadcast by a base station which the cellular phone will receive and combine with the secret PIN code in a pre-defined manner. The cellular phone then transmits a response back to the base station for comparison in and verification by the network. If the cellular phone is roaming outside its home system, the roaming network in the meanwhile challenges the phone's home system with the same random number. If the result transmitted from the cellular phone matches that received from the home system, the cellular phone is admitted to the network and the network is fairly certain that its bill for services can be sent to the phone's home operator and that it will be honored. This procedure, an other similar procedures, are referred to as authentication procedures.
While it may not be too difficult to ensure that unique PIN codes are issued to all cellular phones registered with a particular operator, it is not so obvious how to coordinate PIN code issue between operators to guarantee uniqueness, without compromising security by having too many organizations with access to secret information. Further, it is not very likely at all that coordination could be achieved between continents. Therefore the present invention provides a means of discriminating the security information between different networks such that uniqueness of the information is not a necessity.
U.S. Pat. No. 5,091,942, entitled "Authentication System for Digital Cellular Communication Systems" by Paul Dent, assigned to the same assignee as the present invention incorporated herein its entirety by reference, discloses a bilateral authentication procedure that verifies a mobile phone to the cellular network and vice-versa. The bilateral authentication system disclosed therein also produces, as a byproduct, a temporary variable to be used for scrambling traffic data signals. U.S. Pat. No. 5,060,266, entitled "Continuous Cypher Synchronization for Cellular Communication Systems" by Paul Dent, assigned to the same assignee as the present invention incorporated herein its entirety by reference, describes a type of scrambling system for scrambling data traffic signals. A suitable algorithm for so doing is described in pending U.S. Pat. No. 5,148,485 entitled "Encryption System for Digital Cellular Communications" by Paul Dent, assigned to the same assignee as the present invention incorporated herein its entirety by reference.
The referenced prior art discloses the use of a 64-bit temporary key, which is produced during the authentication procedure, to generate, with the aid of a speech frame or TDMA transmission frame counter, a block of keystream bits for every frame which may be exclusive-ORed to traffic data to prevent the data traffic from being received by a radio not in possession of the same 64 bits.
The known prior art does not provide a means to ensure that different radios, designed to meet the requirements of the same cellular standards, which are delivered to perhaps different continents and accidentally in possession of the same 64-bit key cannot receive or transmit the same signal.